Supercapacitor

A supercapacitor, also known as an ultracapacitor, is a high-capacity energy storage device. It functions as a bridge between two traditional technologies: electrolytic capacitors and rechargeable batteries. While standard solid-state capacitors hold very little energy, supercapacitors provide much higher capacitance. However, they generally have lower voltage limits than those standard capacitors.

A supercapacitor typically stores 10 to 100 times more energy per unit mass or volume than an electrolytic capacitor. They can also accept and deliver electrical charges much faster than batteries. Furthermore, they tolerate many more charge and discharge cycles than rechargeable batteries. This makes them ideal for tasks requiring rapid power bursts rather than long-term storage.

The mechanism of a supercapacitor relies on two primary storage principles. The first is electrostatic double-layer capacitance. This involves the separation of charge in a Helmholtz double layer. This layer forms at the interface between a conductive electrode surface and an electrolyte. The separation of charge occurs in a very tiny space, roughly 0.3 to 0.8 nanometers thick.

In these devices, the mobile charges are solvated ions, known as cations and anions, rather than electrons. The second principle is electrochemical pseudocapacitance. This is an electrochemical storage of energy. It is achieved through Faradaic electron charge-transfer via redox reactions, intercalation, or electrosorption.

These two mechanisms often work together to provide the total capacitance.

Supercapacitors can be classified into different types based on their electrodes. Electrostatic double-layer capacitors (EDLCs) use carbon electrodes or their derivatives. These electrodes achieve a high capacitance through the physical separation of charge. Electrochemical supercapacitors (ECSCs) fall between EDLCs and batteries. These use metal oxide or conducting polymer electrodes. They provide high levels of pseudocapacitance in addition to the double-layer effect.

Some modern versions are even more advanced. For example, lithium-ion capacitors were pioneered by FDK in 2007. These combine an electrostatic carbon electrode with a pre-doped lithium-ion electrochemical electrode to increase capacitance and voltage.

The history of this technology spans several decades of experimentation. In the early 1950s, General Electric engineers experimented with porous carbon electrodes. They were designing fuel cells and rechargeable batteries at the time. In 1957, H. Becker developed a low-voltage electrolytic capacitor with porous carbon electrodes. He noted that the energy storage capacity was extremely high, though he did not yet understand the double-layer mechanism.

In 1966, researchers at Standard Oil of Ohio (SOHIO) developed an electrical energy storage apparatus. By 1978, NEC began marketing these devices as "supercapacitors" for computer memory backup. Brian Evans Conway later performed vital work on ruthenium oxide capacitors. In 1999, he helped define the term "supercapacitor" to include the effects of surface redox reactions.

Significant milestones in development have shaped the modern market. In 1982, the Pinnacle Research Institute developed the first low-resistance supercapacitor for military use. This was marketed as the "PRI Ultracapacitor." Later, Maxwell Technologies adopted the term "Ultracapacitor" for their "Boost Caps" to emphasize power applications. In 1994, David A. Evans developed the "Electrolytic-Hybrid Electrochemical Capacitor." He called these "Capattery" devices. These hybrids combined the high dielectric strength of a tantalum anode with a pseudocapacitive metal oxide cathode.

These could hold about five times more energy than a comparable tantalum electrolytic capacitor.

Today, supercapacitors are used in many practical applications. They are highly effective in systems that require frequent, rapid charge and discharge cycles. In automobiles, buses, trains, and cranes, they are used for regenerative braking. They also provide short-term energy storage and burst-mode power delivery.

On a smaller scale, they serve as power backups for static random-access memory (SRAM). This ensures that data is not lost during sudden power interruptions. Their ability to handle rapid cycles makes them much more durable than batteries in these specific roles.

Understanding supercapacitors requires looking at how they relate to broader electrical systems. The total capacitance of a device depends on how the electrodes are arranged. In a symmetric capacitor, both electrodes have the same capacitance. The total capacitance is then half the value of a single electrode. In an asymmetric capacitor, the total capacitance is determined by the electrode with the smaller capacitance.

As researchers continue to work on improving specific energy and cycle stability, these devices will become even more central to modern technology.